A typical circuit board includes layers of conductive material (e.g., copper) and non-conductive material (e.g., fiberglass) sandwiched together to form a single, rigid board. Some circuit boards include many layers (e.g., more than 20 layers of material).
In general, to manufacture a circuit board having many layers, a circuit board manufacturer creates separate circuit board sheets. Each circuit board sheet typically includes two conductive signal layers and a non-conductive separating layer disposed between the conductive signal layers. The manufacturer typically aligns the circuit board sheets on top of each other and along with other non-conductive sheets (e.g., non-conductive core layers) in an interleaved manner, and laminates the sheets together to form an integrated board, e.g., applies glue, pressure and heat to combine the sheets into the board. Then, among other things, the manufacturer cuts and drills particular features into the board (e.g., mounting holes, notches, etc.), cleans the board, and deposits other features on the exposed surfaces of the board (e.g., plated through-holes, pads, protective coatings, etc.). The manufacturer also mounts circuit board components to the board (e.g., integrated circuits, resistors, capacitors, connectors, etc.) thus forming a completed circuit board.
A typical circuit board includes, as some of the signal layers, ground and power planes for providing power to the circuitry of the circuit board. A ground plane is a signal layer which is generally contiguous in all directions throughout that signal layer, and which is configured to carry a power supply ground signal from an external power supply to the circuit board circuitry (e.g., a reference voltage such as zero volts, chassis ground, etc.). Similarly, a power plane is a signal layer which is generally contiguous in all directions throughout that signal layer, and which is configured to carry a power supply voltage signal from the power supply to the circuit board circuitry (e.g., a DC voltage at a predetermined potential difference from the ground signal).
The typical circuit board further includes other signal layers which do not carry power supply signals for powering the circuit board circuitry, i.e., signal layers which are configured to exclusively carry data signals containing information for controlling the operation of the circuit board circuitry. Such a signal layer typically includes conductive traces. Some of these traces can run individually (i.e., alone), and others can run in sets (e.g., as buses, as differential signal pairs, etc.). For such data signal traces, manufacturers typically attempt to provide line impedances matching the circuit board circuitry so that signal reflection back to the circuitry generating the data signals, and to the receiving circuitry, is minimal.
For example, a signal layer can include differential pair traces which are purposefully spaced a fixed distance apart from each other and from a neighboring ground or power plane so that the impedance between the lines is a fixed and fairly uniform value matching that of the circuitry to which the lines are connected in order to minimize signal reflection. Such traces are further purposefully positioned close to each other so that they share the same exposure to noise (e.g., so that high-frequency differential signals on the lines are exposed to the same interference from components, neighboring traces, etc.).
In some situations, the circuit board includes differential pair traces having a differential impedance of a first value (e.g., 100 Ohms) to accommodate a first type of circuitry (e.g., a processor chipset), and other differential pair traces having another differential impedance of a different value (e.g., 150 Ohms) to accommodate a second type of circuitry (e.g., Fibre Channel devices). One conventional approach to designing a circuit board that includes a signal layer having differential pair traces of different differential impedances is for the manufacturer to initially set (i) the trace widths of all traces to the same trace width value, (ii) the distance between the traces of each differential trace pair to the same trace separation value, and (iii) the distance between the differential trace pairs and the closest ground or power plane to the same layer separation value. At this point, all of the differential pair traces of the design provide a first differential impedance for a first type of circuitry. Next, the manufacturer changes the differential impedance in the design for certain differential pair traces to provide a different differential impedance for a second type of circuitry. To this end, the manufacturer modifies the trace width value and/or the trace separation value for these particular differential pair traces.
For example, a manufacturer can start with a circuit board design exclusively having 50 Ohm traces with some 100 Ohm differential pair traces, and can subsequently convert a few of the 100 Ohm differential pair traces into 150 Ohm differential pair traces. In this example, suppose that the 50 Ohm traces are required to accommodate the majority of circuitry board circuitry (e.g., a processor chipset), and the few 150 Ohm differential pair traces are required to accommodate some other circuitry (e.g., Fibre Channel I/O devices). The manufacturer initially sets the trace width, trace separation, and layer separation values within the design to values that provide 50 Ohm impedance traces and 100 Ohm differential impedance between differential pair traces. Then, the manufacturer converts some of the 100 Ohm differential pair traces within the design into 150 Ohm differential pair traces by decreasing the trace widths and/or increasing the trace distance between differential pair traces to increase the standard impedance of those traces to 75 Ohms or greater (i.e., the impedance between the traces and the closest ground or power plane), and to increase the differential impedance of those differential pair traces to 150 Ohms. The end result is a circuit board having a signal layer that includes both differential pair traces having 100 Ohm impedance (e.g., for the processor chipset circuitry) as well as other differential pair traces having 150 Ohm impedance (e.g., for the Fibre Channel I/O circuitry).